This invention relates to projector lamp assemblies and, more particularly, to lamp and lens arrangements for overhead projectors.
Certain commercially available overhead projectors have used an incandescent projection lamp having a soft glass envelope containing both an internal reflector and an internal condenser lens. A notable disadvantage of such a lamp is the significant reduction in light output over the life of the lamp due to the deposition of tungsten (from the filament) on the plano surface of the internal lens. In addition, the build-up of tungsten deposit on the lens causes internal heating which can eventually result in cracking of the lens and failure of the lamp.
A means for overcoming such problems was required. Since hundreds of thousands of the projectors were in the field, it was desirable that a retrofit assembly be provided which could be installed with relative ease and be compatible optically with the replaced system. By using the word "retrofit" herein, we refer to its meaning as commonly understood in the art, viz., to modify or replace a portion of original equipment to thereby correct the equipment, bring it up to new specifications, or the like. Generally, this modification or replacement takes place sometime after the original equipment has been issued for use or placed on the market.